The field of this invention relates generally to the plasma etching of semiconductor wafers, and more particularly to plasma etch apparatus designed to enhance etch uniformity and to inhibit the undesirable formation and deposition of particulates in the etch environment.
FIGS. 1A and 1B (prior art) illustrate the state of the art with an example of a conventional plasma etch system 20. A sealable chamber or housing 22 is coupled to a vacuum system (not shown) for evacuating the housing. Housing 22 typically has a movable door (not shown) for accessing the interior of housing 22. A pedestal 24 is secured inside housing 22 for receiving a wafer to be processed.
When preparing to perform a plasma etch process, a semiconductor wafer 26 is placed on top of pedestal 24 and clamped securely in place with a clamping ring 28 which generally is formed into the shape of an annulus. Toward its bottom edge, clamping ring 28 is provided with a plurality of inwardly extending fingers 31 used for contacting wafer 26 when clamping wafer 26 in place. Clamping ring 28 has an internal diameter shown in the drawings as a diameter d2, which is somewhat larger than the diameter of the wafer being etched.
One goal during plasma etching is to cover as little of the peripheral surface of wafer 26 as possible. Therefore clamping fingers 31 are designed to minimize their coverage of the wafer periphery and to minimize abberations caused by the fingers contacting the wafer.
Next, a plasma focus ring 30 having an interior diameter d1 and a ring height h1 can be placed over and into contact with clamping ring 28. As discussed below, d1 also approximately equals the diameter of the plasma portion placed proximate to the exposed d2 portion of wafer 26.
Now that the parts are in place inside housing 22, the housing is closed and sealed tightly. A vacuum system evacuates the housing to create a vacuum inside into which a suitable etching gas is introduced. A plasma 32 typically is produced by coupling an RF (radio frequency) power source (not shown) to pedestal 24 according to standard processing methods.
At least a portion of plasma 32 occupies the interior volume defined within plasma ring 30. As is known to these skilled in the art, the plasma has a continuously varying diameter which can be considered to be substantially "fixed" in the sense that the plasma has a time-averaged diameter which is substantially constant. Influences such as magnetic enhancement will make the plasma non-circular at any one point in time, thus causing the diameter to be non-constant and therefore different at any one particular moment in time.
Further, plasma 32 is in close proximity with a top surface 34 of wafer 26. As FIG. 1 shows, plasma diameter d1 is larger than wafer exposed diameter d2. The plasma etch process now begins, with plasma 32 proceeding to etch top surface 34 of wafer 26.
Additionally, it is common practice when performing plasma etching to provide some form of cooling for silicon wafer 26 to keep it from being degraded or otherwise harmed by the heat produced within housing 22. In one arrangement a solid annular washer 23 is inserted between wafer 26 and pedestal 24; these three parts are sealed tightly together to prevent fluid leaks and the parts thereby define a gap 25. An inlet pipe 33, formed so wafer 26 can be placed coaxially with pipe 33 provides fluid communication into and out of gap 25. A cooling or cooled fluid such as a gas 39 is then injected through pipe 33 and into gap 25 where gas 39 provides a thermal conduction path between wafer 26 and pedestal 24.
One problem with present plasma etch techniques is the contamination of etch plasma 32 with contaminants such as particulates or involatile materials that are produced as by-products of the plasma etching process.
For example, a first form of typical contaminants is a plurality of effluents 36 which escape from top surface 34 of wafer 26 during etching. A second form of contaminants is a group of compounds 38 formed in plasma 32 during the etching process, such as particulates or involatile materials created by the chemical bonding of component constituents suspended and moving within the plasma. Effluents 36 and compounds 38 are examples of a species of particulates 40.
Particulates 40 and other involatile materials will adhere to the surfaces of structures residing within housing 22. Plasma contamination is made worse as a result of particulates 40 coming into contact with an interior wall 42 of plasma ring 30. The result is that particulates 40 loosely adhere to wall 42 and may become dislodged onto the wafer when the focus ring is moved. Also, involatile materials may precipitate onto wall 42 to form a film 44.
Film 44 is a source of further contamination due to thermal cycling occurring as an inherent feature of the plasma etch process. Wall 42 and film 44, being composed of different materials, may have different coefficients of thermal expansion and contraction. When film 44 is formed on wall 42, both film and wall are hot.
When silicon wafers are exchanged--that is, a completed processed wafer is removed and replaced with a wafer to be processed--in the process chamber or housing 22, the plasma is turned off, an action which allows the internal surfaces in the housing to be cooled. When this happens, both wall and film contract at different rates because they have different coefficients of expansion. Wall and film are then heated again, and again they expand at different rates. As this cycle is repeated, film 44 can crack and shower particles onto top surface 34 of wafer 26.
FIG. 1C (prior art) shows a reactive gas species concentration distribution occurring across diameter d1. The horizontal or x-axis is a measure of distance along d1 and the vertical or y-axis is a measure of the reactive gas species concentration of that portion of plasma 32. Most desirable is to have a substantially flat concentration profile indicative of uniform reactive gas species concentration, which leads to more uniform etching. However, as shown in regions 48 and 50, etchant gas concentration undesirably increases toward the wafer periphery where the plasma approaches and overlaps the edge of wafer 26.
This cup-shaped concentration profile is undesirable because the result is that wafer etching is less controlled toward the wafer perimeter. The density profile in region 52 is uniform and therefore desirable. The preferred overall profile would extend region 52 by extensions 54 and 56 so the overall density profile would be substantially flat and uniform, thus providing uniform etching over the entire exposed top surface 34 of wafer 26. This improved uniformity can be achieved by enclosing the wafer area with a plasma ring 30. However, as has been shown above, ring 30 by relieving one introduces a new problem of contributing to plasma contamination.
Therefore the technology will be improved if a system can be provided that will lessen or eliminate particulates as a source of pollution in a plasma etch environment and provide a more uniform reactive gas species concentration profile.